MEIOSIS AHL Topic 10.1 IB Biology Miss Werba

DIHYBRID CROSSES & GENE LINKAGE POLYGENIC INHERITANCE AHL TOPIC 10 – GENETICS 10.1 MEIOSIS 10.2 DIHYBRID CROSSES & GENE LINKAGE 10.3 POLYGENIC INHERITANCE J WERBA – IB BIOLOGY 2

THINGS TO COVER Process of meiosis – in detail Process of crossing over & recombination Genetic variety through movement of chromosomes Mendel’s Law of Independent Assortment J WERBA – IB BIOLOGY 3

CHROMOSOMES IN MEIOSIS Command term = DESCRIBE 10.1.1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE Reminder: Homologous chromosomes have the same sequence of genes in the same loci, but slightly different alleles J WERBA – IB BIOLOGY 4

CHROMOSOMES IN MEIOSIS Command term = DESCRIBE 10.1.1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE Homologous chromosomes replicate during the synthesis phase (S-phase) of interphase Yields two pairs of sister chromatids, each joined at a centromere sister chromatids chromatid centromere non-sister chromatids J WERBA – IB BIOLOGY 5

CHROMOSOMES IN MEIOSIS Command term = DESCRIBE 10.1.1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE The homologous pair of chromosomes line up side by side during prophase I. This is called synapsis. J WERBA – IB BIOLOGY 6

CHROMOSOMES IN MEIOSIS Command term = DESCRIBE 10.1.1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE The homologous pair of chromosomes line up side by side during prophase I. This is called synapsis. The pair formed is called a bivalent. bivalent J WERBA – IB BIOLOGY 7

CHROMOSOMES IN MEIOSIS Command term = DESCRIBE 10.1.1, 10.1.2 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE Crossing over may occur between non-sister chromatids during prophase I. Crossing over can lead to recombination of alleles. The crossover points are called chiasmata. chiasma J WERBA – IB BIOLOGY 8

CHROMOSOMES IN MEIOSIS Command term = DESCRIBE 10.1.1, 10.1.2 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE J WERBA – IB BIOLOGY 9

CHROMOSOMES IN MEIOSIS Command term = DESCRIBE 10.1.1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE Meiosis reduces the chromosome number in each nucleus. It forms haploid gametes (n) from diploid somatic cells (2n). J WERBA – IB BIOLOGY 10

CHROMOSOMES IN MEIOSIS Command term = DESCRIBE 10.1.1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE Interphase DNA replication occurs Sister chromatids are formed Growth and other preparation events also occur J WERBA – IB BIOLOGY 11

CHROMOSOMES IN MEIOSIS Command term = DESCRIBE 10.1.1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE Prophase I Chromosomes condense Nuclear membrane disappears Nucleolus becomes invisible Spindle formation Synapsis: homologous chromosomes move side by side form bivalents crossing over may occur(forming chiasmata)  results in new allele combinations , thus increasing variation in gametes J WERBA – IB BIOLOGY 12

CROSSING OVER Command term = OUTLINE 10.1.1 10.1.2 10.1.3 CROSSING OVER Command term = OUTLINE Synapsis Homologous chromsomes pair up, forming a bivalent Crossing over Non-sister chromatids may overlap, forming chiasmata Recombination Alleles are swapped between non-sister chromatids, forming recombinant chromosomes. J WERBA – IB BIOLOGY 13

CROSSING OVER Command term = OUTLINE 10.1.1 10.1.2 10.1.3 CROSSING OVER Command term = OUTLINE Crossing over Increases variation in gametes. Can break up genes that are usually inherited together due to their proximity to each other. These are called linked genes. J WERBA – IB BIOLOGY 14

CHROMOSOMES IN MEIOSIS Command term = DESCRIBE 10.1.1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE Metaphase I Bivalents move to the equator. The chromosomes orientate themselves randomly.  also increases variation in gametes as there are 223 possible combinations! J WERBA – IB BIOLOGY 15

CHROMOSOMES IN MEIOSIS Command term = DESCRIBE 10.1.1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE Anaphase I The spindle fibres contract Homologous pairs are separated One chromosome from each pair goes to each pole Non-disjunction could occur here (ie. failure to separate) and affect the formation of gametes J WERBA – IB BIOLOGY 16

CHROMOSOMES IN MEIOSIS Command term = DESCRIBE 10.1.1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE Telophase I Chromosomes arrive at poles Spindle disappears New nuclei form The cytoplasm divides by cytokinesis. The nuclei are now officially haploid. J WERBA – IB BIOLOGY 17

CHROMOSOMES IN MEIOSIS Command term = DESCRIBE 10.1.1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE Prophase II New spindle forms Forms at right angles to the previous spindle Nuclei break down No crossing over occurs J WERBA – IB BIOLOGY 18

CHROMOSOMES IN MEIOSIS Command term = DESCRIBE 10.1.1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE Metaphase II Sister chromatids align at the equator Spindle fibres attach to the centromeres J WERBA – IB BIOLOGY 19

CHROMOSOMES IN MEIOSIS Command term = DESCRIBE 10.1.1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE Anaphase II Spindle fibres contract Centromeres break Sister chromatids are pulled to opposite poles Non-disjunction could also occur here J WERBA – IB BIOLOGY 20

CHROMOSOMES IN MEIOSIS Command term = DESCRIBE 10.1.1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE Telophase II Chromosomes arrive at poles Spindle disappears Nuclear membrane reappears Nucleolus becomes visible Cytokinesis (cell division) is not part of meiosis but is often considered to be the last stage of telophase II J WERBA – IB BIOLOGY 21

GENETIC VARIATION Command term = EXPLAIN 10.1.3 GENETIC VARIATION Command term = EXPLAIN Genetic variation is increased dramatically as a result of meiosis. Results from: Crossing over in prophase I Random orientation of chromsomes in metaphase I Random fertilisation during reproduction Mate selection J WERBA – IB BIOLOGY 23

INDEPENDENT ASSORTMENT Command term = STATE 10.1.4 INDEPENDENT ASSORTMENT Command term = STATE Mendel’s Law of Independent Assortment: The presence of an allele of one gene in a gamete has no influence over which allele of another gene is present. Only true for unlinked genes ie. those far away from each other J WERBA – IB BIOLOGY 24

INDEPENDENT ASSORTMENT Command term = EXPLAIN 10.1.5 INDEPENDENT ASSORTMENT Command term = EXPLAIN Random orientation of the chromosomes during metaphase I allows the genes to become independently assorted into the gametes. Mendel came up with his ideas before the specifics of meiosis were known (ie. random orientation). Chromosomes and DNA had not been discovered yet. He was looking at the traits themselves and it’s just lucky that he chose two unlinked traits. J WERBA – IB BIOLOGY 25

Sample questions Q1. The diagram below shows chromosomes during meiosis. How many chromosomes and chiasmata are visible? J WERBA – IB BIOLOGY 26

Sample questions Q2. Which processes result in recombination? J WERBA – IB BIOLOGY 27

Sample questions Q3. Outline how meiotic division results in almost an infinite genetic variation in the gametes produced. [2] Q4. State how chromosome number can increase in human beings. [1] J WERBA – IB BIOLOGY 28

Sample questions A1. C A2. A crossing over / chiasmata; shuffles alleles; random orientation of chromosomes; at metaphase I: at metaphase II; 2 max A4. non disjunction J WERBA – IB BIOLOGY 29